U.S. Pat. No. 5,629,980 entitled "System For Controlling the Distribution And Use Of Digital Works", issued May 13, 1997, describes a system which provides for the secure and accounted for distribution of digitally encoded works (hereinafter digital works). However, once a digital work leaves the digital domain, e.g. it is printed out, played or otherwise rendered, it is no longer secure and can be subjected to unauthorized copying. This is a problem for all rendered digital works.
Two known techniques for protecting digital works by imparting information onto the digital work itself are "watermarking" and "fingerprinting". The term watermark historically refers to a translucent design impressed on paper during manufacture which is visible when the paper is held to the light. Because watermarks are impressed using combinations of water, heat, and pressure, they are not easy to add or alter outside of the paper factory. Watermarks are used in making letterheads and are intended to indicate source and that a document is authentic and original and not a reproduction.
One technique for creating such a watermark when a digital work is printed is described in U.S. Pat. No. 5,530,759 entitled "Color Correct Digital Watermarking of Images" issued Jun. 25, 1996. In this approach the watermark image is combined with the digital image to create the watermarked image. The watermark image acts as a template to change the chromacity of corresponding pixels in the digital image thus creating the watermark. In any event, these notices server as social reminders to people to not make photocopies.
The term watermark is now used to cover a wide range of technologies for marking rendered works, including text, digital pictures, and digital audio with information that identifies the work or the publisher. Some watermarks are noticeable to people and some are hidden. In some kinds of watermarks, the embedded information is human readable, but in other kinds the information can only be read by computers.
The term fingerprint is sometimes used in contrast with watermarks to refer to marks that carry information about the end user or rendering event rather than the document or publisher. These marks are called "fingerprints" because they can be used to trace the source of a copy back to a person or computer that rendered the original.
The same technologies and kinds of marks can be used to carry both watermark and fingerprint information. In practice, it is not only possible but often desirable and convenient to combine both kinds of information--for watermarks and fingerprints--in a single mark.
With respect to paper based documents, the simplest approach to providing a mark is a graphical symbol or printed notice that appears on each page. This is analogous to a copyright notice. Such notices can be provided by the publisher in the document source or added later by a printer. These notices serve as social reminders to people to not make photocopies.
Other approaches hide information in the grey codes (or intensity) on a page. Although in principle such approaches can embed data in greycode fonts, their main application so far has been for embedding data in photographs. One set of approaches is described by Cox et al. in a publication entitled "Secure spread spectrum watermarking for Multimedia", NEC Research Institute Technical Report 95-10, NEC Research Institute, Princeton, N.J. 08540. To decode data encoded in the approached described by Cox et al. requires comparing the encoded picture with the original to find the differences. The advantage of these approaches is that they can embed the data in such a way that it is very difficult to remove, not only by mechanical means but also by computational means.
As described above, watermarks need not be perceptible to the viewer. For example, one technique is to embed data in the white space of a document. An example of this kind of approach was described by Brassil, et al. In a publication entitled "Electronic marking and identification techniques to discourage document copying", IEEE Journal on Selected Areas in Communications, Vol. 13, No. 8 pages 1495-1504, October 1995. The idea is to slightly vary the spacing of letters and lines in a digital work. The advantages of this approach are that it is not visible and is hard to remove. A disadvantage is that it has a very limited capacity for carrying data--only a few bytes per page.
Another watermarking scheme for use in digital works representing images is available from the Digimarc Corporation. The Digimarc watermark is invisible and is used to convey ownership information relating to the image. From the Digimarc World Web Page describing their technology (URL http://www.digimarc.com/wt_page.html): "A Digimarc watermark imitates naturally occurring image variations and is placed throughout the image such that it cannot be perceived. To further hide the watermark, the Digimarc watermarking process is perceptually adaptive--meaning it automatically varies the intensity of the watermark in order to remain invisible in both flat and detailed areas of an image." Reading of the Digimarc watermark is through a Digimarc reader which can extract the watermark from the image.
Other related prior art includes Daniele, U.S. Pat. No. 5,444,779, on "Electronic Copyright Royalty Accounting System for Using Glyphs", which discloses a system for utilizing a printable, yet unobtrusive glyph or similar two-dimensionally encoded mark to identify copyrighted documents. Upon attempting to reproduce such a document, a glyph is detected, decoded and used to accurately collect and/or record a copyright royalty for the reproduction of the document or to prevent such reproduction. Furthermore, the glyph may also include additional information so as to enable an electronic copyright royalty accounting system, capable of interpreting the encoded information to track and/or account for copyright royalties which accrue during reproduction of all or portions of the original document.
Merkle, etl al., U.S. Pat. No. 5,157,726 entitled "Document Copy Authentication" describes a system for document authentication which utilizes an ID card coupled to a copying machine capable of reading the ID card. The copying machine imparts digitally encoded identification information, e.g. a digital signature, onto a copied document based on information contained in the ID card. The copied document can then be authenticated by scanning the document to extract and decode the digital signature.